The present invention relates to the general field of turbomachinery for gas turbine aeroengines. It relates more particularly to controlling the clearance between the tips of moving blades of a turbine rotor and a turbine ring of an outlet casing surrounding the blades.
The clearance that exists between the tips of the blades of the turbine and the ring that surrounds them depends on differences between the variations in the dimensions of the rotary portions (disk and blades forming the turbine rotor) and of the stationary portions (outer casing and the turbine ring that it includes). These variations in dimensions are both of thermal origin (associated with variations in the temperature of the blades, of the disk, and of the casing) and also of mechanical origin (in particular associated with the effect of the centrifugal force that acts on the turbine rotor).
In order to increase the performance of a turbine, it is desirable to minimize the clearance so that it is as small as possible. Furthermore, while engine speed is increasing, e.g. on going from an idling speed on the ground to a take-off speed in an aeroengine gas turbine, the centrifugal force acting on the turbine rotor tends to move the tips of the blades towards the turbine ring before the turbine ring has had the time to expand under the effect of the increase in temperature associated with the increase in speed. There is thus a risk of contact at this point of operation, which point is referred to as the pinch point.
It is known to have recourse to a system for actively controlling the clearance at the tips of the blades in a turbine of an aeroengine. A system of that type generally operates by directing air against the outside surface of the turbine ring, which air is taken from a compressor and/or the fan of the engine, for example. Cool air delivered against the outside surface of the turbine ring has the effect of cooling it and thus of limiting its thermal expansion. Clearance is thus minimized. Conversely, hot air encourages thermal expansion of the turbine ring, thereby increasing the clearance and making it possible, for example, to avoid contact occurring at the above-mentioned pinch point.
Such active control is operated by a control unit, e.g. the full authority digital engine controller (FADEC) of the engine. Typically, the control unit acts on an adjustable-position valve in order to control the flow rate and/or the temperature of the air directed against the turbine ring, as a function of a clearance setpoint and of an estimate of the real clearance at the blade tips.
The use of an adjustable-position valve nevertheless presents drawbacks, in particular in terms of cost, of size, and of weight. In addition, controlling the valve requires it to be powered continuously.